Solving This Integral

**trigoon** · December 3rd 2008, 10:51 AM

I'm really having trouble understanding how to solve the following integrals, hopefully someone can help me

∫ (3/(x^2 - 7x +12))

I'm just not sure even how to start. I know that I can take out the constant and make it:

3 ∫ (1/(x^2 - 7x +12))

But what should I do after that, I know that my answer will involve utilizing ln

**running-gag** · December 3rd 2008, 11:12 AM

Hi

You can start by rewriting
$\frac{1}{x^2-7x+12}$ in "simple elements"

using the fact that $x^2-7x+12 = (x-4)(x-3)$

$\frac{1}{x^2-7x+12} = \frac{1}{x-4} - \frac{1}{x-3}$

$\int \frac{3}{x^2-7x+12} \, dx = 3 \int (\frac{1}{x-4} - \frac{1}{x-3}) \, dx = 3\; ln\frac{|x-4|}{|x-3|}$

**trigoon** · December 3rd 2008, 11:21 AM

Originally Posted by running-gag

Hi

You can start by rewriting
$\frac{1}{x^2-7x+12}$ in "simple elements"

using the fact that $x^2-7x+12 = (x-4)(x-3)$

$\frac{1}{x^2-7x+12} = \frac{1}{x-4} - \frac{1}{x-3}$

$\int \frac{3}{x^2-7x+12} \, dx = 3 \int (\frac{1}{x-4} - \frac{1}{x-3}) \, dx = 3\; ln\frac{|x-4|}{|x-3|}$

Thanks! I had you until this part:
$\frac{1}{x^2-7x+12} = \frac{1}{x-4} - \frac{1}{x-3}$

How did you get that?

**running-gag** · December 3rd 2008, 11:36 AM

Originally Posted by trigoon

Thanks! I had you until this part:
$\frac{1}{x^2-7x+12} = \frac{1}{x-4} - \frac{1}{x-3}$

How did you get that?

Experience ...

This is the nominal way to "decompose in simple elements" (translation from French, I do not know if it is called this way in English)

$\frac{1}{(x-a)(x-b)} = \frac{1}{a-b} \,(\frac{1}{x-a} - \frac{1}{x-b})$

**trigoon** · December 3rd 2008, 12:34 PM

Thanks a lot this really helps!

Thread: Solving This Integral

LinkBack

Thread Tools

Display

Solving This Integral

Similar Math Help Forum Discussions

solving this integral

Help solving integral

Solving Integral(s*exp(isr)/(s^2-k^2)) ds

solving a integral with x to the x

Solving an Integral

Search Tags